Transcutaneous energy transfer module with integrated conversion circuity

ABSTRACT

An implantable transcutaneous energy transfer device secondary coil module includes a housing, a secondary coil, power conditioning circuitry, and a low voltage, high power connector. The transcutaneous energy transfer secondary coil is disposed outside the housing and is configured to receive a time-varying magnetic field provided by a transcutaneous energy transfer primary coil, and to convert the time-varying magnetic field into a high voltage, alternating current electric signal within the coil. The power conditioning circuitry is mounted within the housing and is electrically coupled to the secondary coil. The power conditioning circuitry including electronics for converting the high voltage, alternating current electric signal from the secondary coil into a high power, low voltage direct current electric signal. The low voltage, high power connector electrically coupled to the power conditioning circuitry and extending outside the housing for connecting the secondary coil module to a power bus for delivering power to implanted devices.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/316,138, filed Dec. 10, 2002, entitled “Transcutaneous EnergyTransfer Module with Integrated Conversion Circuitry,” naming asinventors Farhad Zarinetchi and Robert M. Hart, and now currentlypending, which application is a continuation-in-part of U.S. patentapplication Ser. No. 09/957,330, filed Sep. 20, 2001, and now U.S. Pat.No. 6,496,733, entitled “Transcutaneous Energy Transfer Device withMagnetic Field Protected Components in Secondary Coil,” naming asinventors Farhad Zarinetchi, Robert M. Hart, Michael C. Verga, andStephen J. Keville; which application is a divisional of U.S. patentapplication Ser. No. 09/346,833, filed Jul. 2, 1999, and now U.S. Pat.No. 6,324,431; which application is a continuation-in-part of U.S.patent application Ser. No. 09/110,607 filed Jul. 6, 1998 entitled “TETDevice with Magnetic Field Protected Components in Secondary Coil,” andnaming as inventors Farhad Zarinetchi and Robert M. Hart, now abandoned.

FIELD OF THE INVENTION

This invention relates to transcutaneous energy transfer (TET) devicesand, more particularly, to such a device which includes powerconditioning circuitry within a secondary coil.

BACKGROUND OF THE INVENTION

Many medical devices are now designed to be implantable includingpacemakers, defibrillators, circulatory assist devices, cardiacreplacement devices such as artificial hearts, cochlear implants,neuromuscular simulators, biosensors, and the like. Since almost all ofthe active devices (i.e., those that perform work) and many of thepassive devices (i.e., those that do not perform work) require a sourceof power, inductively coupled transcutaneous energy transfer (TET) andinformation transmission systems for such devices are coming intoincreasing use. These systems consist of an external primary coil and animplanted secondary coil separated by an intervening layer of tissue.

One problem encountered in such TET systems is that the best place tolocate control circuitry for converting, conditioning, amplifying orotherwise processing the signal received at the secondary coil beforesending the signal on to the utilization equipment is within thesecondary coil itself. However, there is also a significant magneticfield in the secondary coil resulting from the current induced therein,which field can induce heating of the components, particularly metalliccomponents. At a minimum, such heating can influence the performance ofvarious components, and in particular interfere with the desired uniformpower applied to the equipment. In a worst case, the heating can besevere enough to cause damage or destruction to the components, whichcan only be repaired or replaced through an invasive surgical procedure.Such heating can also cause injury or discomfort to the patient in whichthe components have been implanted.

Heretofore, in order to avoid such heating, it has either been necessaryto be sure that the signal induced in the secondary coil is notsufficient to generate a time-varying magnetic field which would causepotentially damaging heating of the components or to mount thecomponents at a less convenient location. The former is undesirablebecause it is generally not possible to eliminate significant heating ofthe components while still operating the device at required energylevels, and the later solution is not desirable since the output signalfrom the secondary coil can reach 500 volts and above at an operatingfrequency that can be in excess of 100 kHz. It is preferable that suchhigh voltage signal not pass extensively through the body and it isdifficult to provide good hermetically sealed connectors for signals atthese voltages. In addition, such high frequency signals can causeelectrical interference with other electrical systems that may beimplanted—such as, for example, an implanted controller for controllinga blood pumping device that is being powered by the TET system. It istherefore preferable that an auxiliary signal processing module, whichmay reduce the voltage to a value in the approximately 20 volt range, beincluded as close to the secondary coil as possible, a position insidethe secondary coil being ideal for this purpose.

A need therefore exists for an improved technique for use with TETdevices so as to enable at least selected electronic components to bemounted within the secondary coil with minimal heating of such devices.A need further exists for TET devices coupled to low voltage, high powerbuses for distributing power to distributed implanted devices and highpower implanted devices such as blood pumping devices.

SUMMARY OF THE INVENTION

In accordance with the above, one aspect of the invention provides animplantable transcutaneous energy transfer device secondary coil modulefor receiving and conditioning power from a time-varying magnetic fieldprovided by a transcutaneous energy transfer primary coil. The secondarycoil module of the invention includes an inner housing, a secondarycoil, power conditioning circuitry, and a low voltage, high power outputmeans. The transcutaneous energy transfer secondary coil is disposedoutside the inner housing and is configured to receive a time-varyingmagnetic field provided by a transcutaneous energy transfer primarycoil, and to convert the time-varying magnetic field into a highvoltage, alternating current electric signal within the coil. The powerconditioning circuitry is mounted within the inner housing and iselectrically coupled to the secondary coil. The power conditioningcircuitry includes electronics for converting the high voltage,alternating current electric signal from the secondary coil into a highpower, low voltage direct current electric signal. The low voltage, highpower output means is electrically coupled to the power conditioningcircuitry and extends outside the module for connecting the secondarycoil module to a power bus for delivering power to implanted devices.

In a further aspect of the invention, an implantable blood pump energysupply system is provided having a transcutaneous energy transfer devicesecondary coil module, an implantable power bus electrically coupled tothe secondary coil module, and a blood pumping device electricallycoupled to and receiving power from the power bus. The secondary coilmodule has a inner housing, a secondary coil disposed outside the innerhousing, power conditioning circuitry mounted within the inner housingand electrically coupled to the secondary coil, and a direct currentconnector electrically coupled to the power conditioning circuitry andextending outside the module. The secondary coil is configured toreceive a time-varying magnetic field provided by a transcutaneousenergy transfer primary coil and convert the time-varying magnetic fieldinto an alternating current electric signal within the coil. The powerconditioning circuitry includes electronics for converting analternating current electric signal from the secondary coil into adirect current electronic signal.

In a still further aspect of the invention, an implantabletranscutaneous energy transfer device secondary coil module having a lowvoltage output is provided for receiving a time-varying magnetic fieldfrom a transcutaneous energy transfer primary coil and converting thetime-varying magnetic field to a low voltage output. The module includesan inner housing, a secondary coil, power conditioning circuitry, and alow voltage, high power output means. The transcutaneous energy transfersecondary coil is disposed outside the inner housing and is configuredto receive a time-varying magnetic field provided by a transcutaneousenergy transfer primary coil and to convert the time-varying magneticfield into a high voltage, alternating current electric signal withinthe coil. The power conditioning circuitry is mounted within the innerhousing and is electrically coupled to the secondary coil. The powerconditioning circuitry includes electronics for converting analternating current electric signal from the secondary coil having ahigh voltage greater than or equal to about 200 volts into a directcurrent electric signal having a low voltage of less than or equal toabout 50 volts and a high power level of greater than or equal to about15 watts. The low voltage, high power output means is electricallycoupled to the power conditioning circuitry and extends outside theinner housing for connecting the secondary coil module to a power busfor delivering power to implanted devices.

In another aspect, the invention provides a transcutaneous energytransfer system having a primary coil and a secondary coil module. Theprimary coil is adapted to be placed outside a patient for providing atime-varying magnetic field that passes into the patient. The secondarycoil module is adapted to be implanted within the time-varying magneticfield within the patient provided by the primary coil. The secondarycoil module has an inner housing including an inductive heat reducingmeans; a transcutaneous energy transfer secondary coil disposed outsidethe housing and configured to receive the time-varying magnetic fieldprovided by the transcutaneous energy transfer primary coil and convertthe time-varying magnetic field into a high voltage, alternating currentelectric signal within the coil; and power conditioning circuitrymounted within the housing and electrically coupled to the secondarycoil, the power conditioning circuitry including electronics forconverting a high voltage, alternating current electric signal from thesecondary coil into a high power, low voltage direct current electricsignal. An output line is electrically coupled to the power conditioningcircuitry and extends outside the module to transmit the high power, lowvoltage direct current electric signal outside of the module.

In specific embodiments of the various aspects of the invention, thesecondary coil module inner housing includes an inductive heat reducingmeans. In further specific embodiments, the inductive heat reducingmeans can include a cage formed of magnetically permeable material andthe magnetically permeable material can have a magnetic permeability ofbetween about 2000 and 5000. The cage can also include walls having athickness adapted to maintain magnetic flux within the magneticallypermeable material below saturation. The inductive heat reducing meanscan also be configured to maintain a total inductive heat dissipationfrom the power conditioning circuitry below about 150 milliwatts, evenwhere the high voltage, alternating current electric signal in thesecondary coil has a voltage of greater than or equal to about 500 voltsand a frequency greater than or equal to about 200 kHz. Still further,the inductive heat reducing means can include a counterwound coil.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is pointed out with particularity in the appended claims.The above and further advantages of this invention may be betterunderstood by referring to the following description when taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a schematic semi-block diagram of an exemplary TET system;

FIG. 1A is a schematic diagram of a TET system and power bus of theinvention;

FIG. 1B is an interface map showing the electronics from the TET systemof FIG. 1A;

FIG. 1C is a diagram of a TET system according to the invention in whicha housing containing power conditioning electronics is adjacent orproximate to a secondary coil within a secondary coil module wherein thehousing and secondary coil are both within a time-varyingelectromagnetic field created by a primary coil;

FIG. 2 is a cutaway sectional view of the secondary coil and electronicsfor a system of the type shown in FIGS. 1 and 1A in accordance with anembodiment of the invention;

FIGS. 3A and 3B are a side-sectional view and a top plane viewrespectively of a secondary coil and electronics in accordance withanother embodiment of the invention;

FIG. 3C is a top view for an alternative of the embodiment illustratedin FIGS. 3A and 3B;

FIGS. 4A, 4B and 4C are side-sectional views of the secondary coil andelectronics illustrating the magnetic field lines at the secondary forthe embodiment shown in FIG. 1, FIG. 2, and FIGS. 3A-3C respectively;

FIG. 5 is a cross-sectional view of an alternative embodiment of a cageof the present invention having flanges or extensions extendingtherefrom;

FIG. 6 is a simplified cross-sectional view of a cage in accordance withone embodiment of the present invention; and

FIG. 7 is a cross-sectional view of an implantable device having asecondary coil in accordance with an alternative embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an exemplary TET system 10 is shown which includesa primary coil 12 connected to receive an alternating drive signal froman appropriate drive circuit 14 and a transcutaneously mounted secondarycoil 16 having signal processing electronics 18 mounted therein. Thesignal from electronics 18 is applied to operate a utilization device 20which may for example be a blood pump, artificial heart, defibrillator,or other implanted device requiring power or other signals appliedthereto.

For a number of reasons, it is desirable for electronics 18 to includepower conditioning circuitry—such as, for example, rectifying orconverting circuitry, regulating circuitry, monitoring circuitry, orother circuitry known for use in modifying power signals, within asecondary coil module. In the exemplary system diagram of FIG. 1A, asecondary coil module 24 is illustrated as including an inner housing 26with the secondary coil 16 provided outside the inner housing. Secondaryelectronics 18 are provided inside inner housing 26 and areelectronically coupled to low voltage line 32 and low voltage connector34. Secondary coil module 24 can be encased in an outer housing toprovide a sealed housing for implantation within a patient (see, e.g.,outer housing 702 below), for example by overmolding the module with anepoxy and may further be coated with a biocompatible agent or have avelour material attached. Connector 34 preferably extends outsidesecondary coil module 24, meaning that connector 34 can extend outsideof inner housing 26, and, if present, an outer housing of the module 24.

A high power, low voltage bus 36 can be connected to low voltageconnector 34 to bus power to other portions of an implanted system suchas battery 38 (which may use power to charge the battery or providepower when operating in battery power mode, or some combinationthereof), controller 40, and a high power medical device such as a bloodpumping device or a total artificial heart. A person of ordinary skillin the art will recognize that more or fewer components may be connectedto low voltage bus 36, for example, power could be provided to highpower medical device 42 through controller 40, or battery 38 andcontroller 40 could be integrated into the same module.

For providing power for use by a high power implanted medical device 42,approximately 15 to 50 watts of power will typically be supplied on bus36, or more specifically, about 20 watts. In order to supply this levelof power to high power implanted medical device 42, high voltages (500to 1000 volts) and high frequencies (200 kHz or higher) are produced insecondary coil 16. FIG. 1B provides an exemplary interface map forsecondary coil module 24 to further illustrate the secondary electronics18 contained therein. This figure shows a secondary coil 16 electricallycoupled to secondary electronics 18, which in this case includes a fullwave rectifier 44 and a filter 46, and leading to low voltage output 32.In the illustrated embodiment, secondary electronics 18 also includes aheat sensor 48 and an interface 50 to a communication bus forcommunicating sensor information to implanted controller 40. The outputprovided on low voltage output 32 is generally less than or equal toabout 50 volts at the desired power levels, or more specifically, about30+/−2 volts at a constant output of about 35 watts, or between about 24and 45 volts for a pulsating load between about 0 and 45 watts.

In one exemplary embodiment, secondary coil 16 consists of 12 turns ofLitz wire in a three-inch diameter coil. Secondary coil 16 can also beconnected to a series-resonant tank circuit and a rectifier circuit thatconverts the RF power output of the tank circuit to DC power. Therectifier circuit may utilize Schottky diodes in a bridge configurationor use another rectifying circuit known in the art. An autoregulationcircuit, such as a buck type DC/DC converter that regulates the outputof the rectifier to a desired DC voltage may also be included. Ofcourse, a person of ordinary skill in the art will be aware of other ordifferent circuits that could achieve the desired output.

A system of the invention having conversion electronics incorporatedwithin secondary coil module 24 to provide a single integrated secondarymodule having a low voltage, high power output solves a number ofproblems in the art. Placing the conversion electronics within thesecondary coil module instead of other modules reduces the amount ofelectrical noise (note the frequencies and voltages produced in thesecondary coil as described above) that might interfere with moresensitive electronic components such as controller 40.

Placing the conversion electronics within the secondary coil module asopposed to within other modules in a distributed implanted medicalsystem also increases the reliability of the various modules as theconversion electronics dissipate a large amount of heat, raising theambient temperature (and thus lowering the reliability) of the otherelectronics that share a module with it. In addition, providingconversion at the secondary coil module allows all connectors in thesystem to be smaller low voltage connectors, and further eliminates anyneed for high voltages to be transmitted on implanted cables through thebody. Still further, placing the conversion electronics within thesecondary coil module means that the conversion electronics do not haveto be packaged in a separate module, which would further have to beimplanted within the patient. While the illustrative embodiment showselectronics 18 surrounded by secondary coil 28, the advantages of theinvention also apply where electronics 18 are proximate or adjacent tosecondary coil 28, in particular, where both the secondary coil and theelectronics are provided within the time-varying magnetic field providedby primary TET coil 12 as is illustrated in FIG. 1C (while theembodiment in FIG. 1C illustrates a side-by-side proximity, a person ofordinary skill in the art will recognize that other configurations,including vertical displacement between the secondary coil and thehousing/electronics). In such an embodiment, high voltage, highfrequency signals are not transmitted through the body, and innerhousing 26 protects electronics 18 from inductive heating fromtime-varying magnetic field, thus providing the advantages of theinvention without the disadvantages of prior art approaches.

A further advantage of the system of FIG. 1A is that the low voltageconnector 34 and low voltage power bus 36 provide the system withgreater reliability and with greater ability to repair modules withoutdisturbing other modules or their power requirements. For example, ifthe power electronics 18 or secondary coil 28 itself were to fail,secondary coil module 24 could readily be swapped out, or the bus 36powered by providing a DC power source to connector 34 so that power canbe maintained or supplemented even where secondary coil module 24 mustbe removed for a period of time.

Despite these many advantages, no implanted TET system for transmittinghigh power to devices such as ventricular assist or artificial heart(i.e., blood pumps) has successfully included conversion electronicswithin a secondary coil module as described by the present inventors.This is true primarily because for high power applications (notincluding low power applications such as pace makers, cochlear implants,or trickle charging, each of which typically operate using onlymilliwatts (less than a watt, and typically less than a tenth of a watt,of power) the alternating magnetic field that transmits power to thesecondary coil causes severe inductive heating within any electronicsplaced within the coil, leading to patient tissue damage and/or failureof the electronics. (See, e.g., Weiss et al., “A Completely ImplantedLeft Ventricular Assist Device, Chronic In Vivo Testing,” ASAIO Journal,Vol. 39(3), pp. M427-M432 (July-September 1993) where power conditioningelectronics were move from the secondary module to prevent tissuedamage).

As illustrated in FIG. 4A, the magnetic field lines generated by thesecondary coil 16 pass through the electronics 18, resulting in heatingof the electronics and, in particular, metal portions thereof. This cancause undesired variations in the outputs from this device or, worstcase, in component failure. In accordance with certain aspects of thepresent invention, FIGS. 2 and 3A-3C illustrate two techniques forreducing inductive heating of such electronic components 18 by themagnetic field generated by the primary and secondary coils. Referringto FIG. 2, the electronics 18 are fully enclosed within an inner housing26 comprising a cage 22 formed of a high magnetic permeability material,the material for cage 22 preferably having a magnetic permeability inthe range of approximately 2000-5000. Cage 22 is formed of a base 22Aand a lid 22B which are fitted and held together in a manner known inthe art which minimizes interruption of magnetic field lines passingthrough the cage. For a preferred embodiment, the material utilized forthe cage is MnZn ferrite (Phillips 3F3), although other high magneticpermeability materials now or later developed may be used. The materialis preferably one formulated for high frequency application with lowpower loss. Since it is desirable that cage 22 be as light (have minimalmass) as possible, the wall thickness (t) of the cage should be nothicker than is required to protect electronics 18 therein, preferablywhile minimizing heat loss. In particular, the thickness should be theminimum thickness for the material that provides magnetic field valuesin the material which are well below saturation, saturation preventingthe flux from being guided effectively, and below the level that wouldlead to significant (for example, greater than 100 mw) heat dissipationin the material under operating conditions. Saturation in the cage is afunction of magnetic field strength, while heat generation is a functionof both magnetic field strength and frequency. For a ferrite cagematerial, the field strengths and frequencies at which these effectsoccur can be tabulated. With a given set of operating conditions, fieldstrength or flux density increases with decreasing wall thickness. Themagnetic field strength B may be determined by measurement at the topsurface of the cage 22, and the total flux Φ_(t) may be determined asthe integral of B over the surface. This flux is guided through thesidewalls of the cage, density being highest in these sidewalls. Themagnetic flux in the walls may then be calculated as the total fluxdivided by the cross sectional area of the wall:

$B_{\max} = \frac{\Phi_{t}}{\left( {\pi \; {Dt}} \right)}$

A thickness t may be chosen so that B_(max) is below the saturationlevel. In an alternative embodiment, the thickness t is also chosen sothat the total heat dissipation at the operating frequency is less than100 mw. In another embodiment, the thickness t is also chosen so thatthe total heat dissipation at the operating frequency is between 50 and100 mw. In a still further embodiment, the thickness t is also chosen sothat the total heat dissipation at the operating frequency is less than150 mw. In another embodiment, the thickness t is also chosen so thatthe total heat dissipation is between 70-90 mw. Still further, theseheat dissipation goals may be achieved at operating conditions such asthose described herein, for example, with secondary coil voltage greaterthan or equal to about 500 volts, secondary coil frequency at greaterthan or equal to about 200 kHz, and/or including the provision of atleast 15 watts output at less than or equal to 50 volts.

For an exemplary embodiment using the material previously indicated,wall thickness (t) is approximately 0.06 inches, and the diameter (D) ofthe cage for this embodiment is approximately 1.9 inches. However, thesedimensions may vary significantly with application, for example the sizeof secondary coil 16, the electric energy applied thereto and the like.Further, the thickness of the cage need not necessarily be sufficient todivert all magnetic flux lines from the electronic 18 so long as it iseffective to divert sufficient magnetic flux lines so as to prevent anysignificant heating of the electronic components. FIG. 4B illustratesthe magnetic field for secondary coil 16 when a cage 22 of the typeshown in FIG. 2 is employed. From FIG. 4B, it can be seen that themagnetic field is concentrated in the high permeability material of thecage so that the field at the electronic components 18 is reduced tonearly zero, with the magnetic field concentration being higher in thesidewalls of the cage than in the top and bottom walls. Thus, overallcage thickness and weight may in some instances be reduced by making thesidewalls thicker to accommodate the flux therein, with the top andbottom walls of the cage being thinner.

In one preferred embodiment, the optimal thickness of the cage wall hasthe lightest weight (lowest mass) without causing excessive heatdissipation. To have the lightest cage 22, the cage wall thickness isreduced, resulting in an increase in the magnetic flux density, B. Tosatisfy operation without excessive heat dissipation, B must be keptbelow the saturation density, B_(max), for the selected cage material.Should B_(max) be exceeded, the heat dissipation of the materialincreases dramatically. The combination of these two requirements, then,results in the desired requirement that B₀=B_(max), where B₀ is the fluxdensity at the highest possible magnetic field strength for the device.

B₀ at any given point along the cage can be manipulated by varying thecage wall thickness. The design consideration is to reduce the crosssectional area of the cage so that B₀ is less than or equal B_(max)along a substantial portion of the cage wall. An example of an algorithmfor this process for a cylindrically symmetric cage includes firstassuming a uniform thickness for the cage wall. Then, the total flux forhighest power transfer within the cage walls, i.e. Φ₀ using therelationship B₀=Φ₀/Lt, where L is the perimeter of the cage 22 at thepoint of interest and t is the cage thickness wall. Given that L isdetermined by other design considerations, t can be adjusted so thatB₀=B_(max).

While for certain embodiments, the material of cage 22 is of a ferritematerial, other high permeability materials might also be utilized suchas, for example, laminated iron materials. For ease of fabrication, itmay also be desirable to form the cage as three or more distinctsegments, for example a top and bottom disk with a cylinder for thesidewalls, the disks and sidewalls being held together by a suitableepoxy, solder or other suitable means known in the art. This form offabrication may be particularly desirable where the sidewalls are of adifferent thickness than the top and bottom walls. Preferably, the breakin material continuity is minimized to avoid a significant reduction inthe efficiency at which the magnetic field is conducted through thecage. Finally, forming the cage of alternate layers of ferrite materialwith high thermal and magnetic conductivity and epoxy or other similarmaterial with low thermal and magnetic conductivity, results in a cagewhich is more anisotropic for magnetic flux flow, and thus providespotentially better flux guidance. In particular, such constructionprovides a lower reluctance path for the magnetic flux and magneticfields through the ferrite layers then in a direction perpendicularthereto.

FIGS. 3A and 3B illustrate an alternative embodiment of the inventionwherein reduced magnetic field at electronic components 18 is achievedby dividing the secondary coil into an outer coil 16A and an inner coil16B which is counter-wound with the coil 16A so as to provide anopposing field. In this embodiment, the two sets of coils are connectedso that the same current flows in each, and the secondary coil moduleinner housing in this embodiment can be any structure that serves tohold electronic components 18 within inner coil 16B. The relativediameters of the two coils, D₁ and D₂ and the number of turns for thecoils, N₁ and N₂, are adjusted so that their separate contributions tothe total magnetic field at the location of components 18 significantlycancel each other. In achieving this objective, the field generated inthe central region by a flat outer coil 16A is given by:

$B_{1} = \frac{\mu \; N_{1}i}{D_{1}}$

while the field generated by the inner coil 16B in the central region ofthe coil is given by:

$B_{2} = \frac{\mu \; N_{2}i}{D_{2}}$

When the strengths of these two fields are selected to be equal, theratio between the outer and inner windings becomes:

$\frac{N_{1}}{N_{2}} = \frac{D_{1}}{D_{2}}$

Using these criteria with an illustrative embodiment where D₁=2.5inches, D₂=1.5 inches and N₁=19, a value for N₂=11.4 would be obtained.However, for this implementation, because the inner coil was slightlyelongated along the field direction, a preferred value for N₂ was foundto be 7. Therefore, while the four values (the N and D values) can becalculated to achieve the desired magnetic field cancellation for agiven application, it has been found to be easier and more accurate toselect the parameters empirically, for example by selecting three of theparameters to achieve substantial field cancellation and then adjustingthe fourth parameter until the field at the components 18 has beenreduced so that there is no significant heating of the metal componentsthereof. Thus, assuming the other three values are given, an N₂ might bedetermined as follows:

1. Wind the outer coil with the predetermined number of turns (19 in theexample given).

2. Insert a magnetic field monitoring device such as a gauss meter inthe central region of the coil.

3. Apply a dc current through the outer coil and monitor the magneticfield in the central region.

4. Locate one of the ends of the outer coil and using the wire extensionfrom this coil, start winding the inner coil in the opposite directionof the outer coil.

5. As the inner coil is wound, monitor the strength of the magneticfield in the central region. Stop winding the inner coil when this fieldreaches zero.

Other empirical procedures might similarly be used for determining theparameter values in order to achieve substantial field cancellation atthe center of the coil. For example, the inner coil could be wound andthe system can be tested at operating conditions to determine whetherthe magnetic field measured would result in heat dissipation in thesystem that is less than or equal to 150 mw, 100 mw, or 90 mw asdescribed above for the ferrite box embodiment.

FIG. 4C illustrates a magnetic flux pattern that might be achieved withthe winding pattern of FIGS. 3A-3B. From FIG. 4C, it can be seen thatthe magnetic field at components 18 can be reduced to substantially zeroutilizing this technique. However, since this technique involvessignificant signal cancellation in the secondary coil, it results inreduced energy transfer efficiency for the device. It may therefore notbe suitable for use in applications where high-energy transferefficiency is required.

FIG. 3C illustrates an alternative to the embodiment shown in FIG. 3Bwhich to some extent reduces the loss of energy transfer efficiencyresulting from the counterflowing current in coil 16B. For thisembodiment, coil 16B′ is a few turns of wire, for example one or two,having the diameter D₂ but not electrically connected to the windings16A′. The magnetic flux resulting from current flow in windings 16A′induce a current in winding 16B′ which in turn produce a magnetic fieldcountering that generated by that winding 16A. By suitable selection ofboth the number and diameter of the windings 16A′ and 16B′, fieldcancellation such as that shown in FIG. 4C can be achieved.

In an alternative embodiment, the cage of the present invention isconfigured to increase the permeability in a flux pathway between theprimary and secondary coils that is closest to the secondary coil. Inone embodiment this is achieved by extending the high permeabilityshield material of cage 22 to a location within the flux pathway. Thisincreases the total permeability of the flux pathway with acorresponding increase in coupling between the primary and secondarycoils.

An exemplary implementation of this alternative embodiment of the cageis illustrated in FIG. 5. As shown therein, the inner housing comprisesa cage 500 including a base 502 and a lid 504. In this illustrativeembodiment, base 502 is cylindrical while lid 504 is shaped in the formof a disk. Base 502 includes and integral flange 506 that extends thecage material from base 502 into the magnetic flux pathway 508.

To increase the coupling between the primary and secondary coils, flange506 is preferably in-line with the shortest flux pathway between theprimary and secondary coils. In other words, flange 506 extends frombase 502 immediately adjacent to secondary coil 512 to guide themagnetic flux lines back toward the primary coil. The extent to whichthe flange 506 extends away from base 502 is based on the mass andvolume limitations of the device. However, flange 506 preferably doesnot have a thickness less than that which would cause saturation of themagnetic material, particularly at highest primary field strengths.

It should be understood that flange 506 may be the same or differenthigh permeability material as base 502. In devices wherein flange 506and cage 22 are of the same material, flange 506 and base 502 arepreferably formed as a unitary device. However, in alternativeembodiments, flange 506 may be attached to base 502 using well-knowntechniques.

FIG. 6 is a simplified cross-sectional view of an alternative embodimentof a cage 600. Cage 600 includes a base 602 and a self-aligning orinterlocking lid 604. In this illustrative embodiment, base 602 iscylindrical while lid 604 is shaped in the form of a disk. Base 602includes vertical walls 606 onto which lid 604 is attached, typically bybonding.

Lid 604 includes an annular recessed shelf 608 circumferentially formedaround mating surface 610 of lid 604. Shelf 608 is configured to receivevertical wall 606 of base 602 thereby preventing relative motion of lid604 and base 602 during assembly. It should be understood that otherfeatures may be used to align and/or secure lid 604 with base 602, suchas by threading, etc. Alternatively, mating surface 610 of lid 604 mayhave a track or groove configured to accept base vertical wall 606. Itshould also be noted that annual recessed shelf 608 may be configured asa square or any other shape to accept base 602.

FIG. 7 is a cross-sectional view of an implantable device outer housing702 enclosing a secondary coil 704. Inner housing or cage 706 ispositioned within secondary coil 704, as described above. Cage 706serves to house electronic components 708 that generate heat,illustrated by arrows 710. In accordance with this embodiment of thepresent invention, heat in implanted device 702 is distributed to avoidlocalized high temperature regions as well as tissue necrosis.

Outer housing 702 is constructed of a relatively low thermalconductivity medium such as potting epoxy. Cage 706 preferably includesflange 712 similar to the flange described above with reference to FIG.6. In accordance with this embodiment of the present invention, a heatdistribution layer 714 is thermally coupled to cage 706 and outerhousing 702. Heat generated by electronics 708 is transferred viaconduction to cage 706. To facilitate the transfer of such heat intodistribution layer 714 good thermal contact is maintained betweendistribution layer 714 and cage 706 using a thermally conductive medium.In addition, the largest contact area between distribution layer 714 andcage 706 suitable for the implemented design is implemented. Forexample, in the embodiment illustrated in FIG. 7, flange 712 providesbase 715 with the greatest area over surface 720 as compared to othersurfaces of cage 706. Accordingly, distribution layer 714 is thermallycoupled to cage 706 at surface 720. Similarly, distribution layer 714 ispreferably thermally coupled to a substantial area of outer housing 702.This provides for the substantially uniform distribution of heat alongouter housing 702. In the embodiment illustrated in FIG. 7, distributionlayer 714 is thermally coupled to the entire housing 702 other than theportion of outer housing 702 adjacent bottom surface 720 of cage 706.

In one embodiment, distribution layer 714 has a high thermalconductivity and a low magnetic permeability and low electricalconductivity. For example, in one preferred embodiment, distributionlayer 714 is formed from alumina powder suspended in an epoxy medium.The thickness of the distribution layer 714 is preferably in the rangeof 2-10 mm, although other thicknesses and materials may be used for agiven application. In an alternative embodiment, distribution layer 714is comprised of multiple alternating layers of high and low heatconductivity materials.

While specific embodiments have been disclosed above for reducing themagnetic field at the center of a TET device secondary coil so as topermit device electronics to be mounted in the coil without excessiveheating, it is to be understood that other techniques for achieving thisobjective and/or other variations on the embodiments disclosed arewithin the contemplation of the invention. Thus, while the invention hasbeen particularly shown and described above with reference to preferredembodiments, the foregoing and other changes in form and detail may bemade therein by one skilled in the art while still remaining within thespirit and scope of the invention, which is to be defined only by thefollowing claims. In particular, several of the embodiments disclosedherein can be combined, For example, the ferrite box, ferrite box withflange, and counterwound coil features can be combined. By combiningfeatures and measuring the magnetic flux in at the electronics, a personof ordinary skill in the art can select from among these features toproduce a system in which a desired heat dissipation maximum can beachieved at operating conditions.

What is claimed is:
 1. An implantable transcutaneous energy transferdevice secondary coil module for receiving and conditioning power from atime-varying magnetic field provided by a transcutaneous energy transferprimary coil, comprising: a transcutaneous energy transfer secondarycoil comprising an inner winding and an outer winding, the secondarycoil configured to receive the time-varying magnetic field provided bythe transcutaneous energy transfer primary coil and convert thetime-varying magnetic field into a high voltage, alternating currentelectric signal within the secondary coil; and power conditioningcircuitry disposed in a central region of the inner and outer windingsand electrically coupled to the secondary coil, the power conditioningcircuitry including electronics for converting a high voltage,alternating current electric signal from the secondary coil into a highpower, low voltage direct current electric signal for powering implanteddevices, wherein a magnetic field generated by the inner winding and amagnetic field generated by the outer winding when an electrical currentis applied to at least one of the inner and outer windings aresubstantially opposed to one another at the central region.
 2. Themodule of claim 1, wherein the magnetic fields generated by the innerand outer windings substantially cancel one another at the centralregion.
 3. The module of claim 1, wherein the inner and outer windingsare configured to maintain a total inductive heat dissipation from thepower conditioning circuitry below about 150 milliwatts.
 4. The moduleof claim 3, wherein inductive heat dissipation is maintained below about150 milliwatts under operating conditions wherein the high voltage,alternating current electric signal has a voltage of greater than equalto about 500 volts and a frequency greater than or equal to about 200kHz.
 5. The module of claim 1, wherein the inner and outer windings areelectrically connected.
 6. The module of claim 5, wherein the innerwinding is counterwound relative to the outer winding.
 7. The module ofclaim 1, wherein the inner and outer windings are electrically isolatedfrom each other.
 8. The module of claim 7, wherein the inner winding iscounterwound relative to the outer winding.
 9. The module of claim 7,wherein magnetic flux resulting from current flow in the outer windinginduces a current in the inner winding and the current induced in theinner winding generates the magnetic field of the inner winding.
 10. Themodule of claim 1, wherein the power conditioning circuitry is containedwithin an inner housing disposed in the central region.
 11. The moduleof claim 10, further comprising a heat distribution layer providedexternally to the module and a thermally conductive medium providingthermal conduction between the inner housing and the heat distributionlayer.
 12. The module of claim 10, wherein the inner winding is adjacentthe inner housing and disposed between the inner housing and the outerwinding.
 13. The module of claim 1, wherein the high power, low voltagedirect current electric signal has a voltage less than or equal to about50 volts and a power level greater than or equal to about 15 watts. 14.The module of claim 1, wherein the high voltage, alternating currentelectric signal is greater than or equal about 200 volts.
 15. The moduleof claim 1, further comprising a power connector coupled to the powerconditioning circuitry and accessible from outside the module forconnecting the secondary coil module to a power bus for delivering powerto implanted devices.
 16. The module of claim 15, further comprising animplantable power bus connected to the power connector.
 17. The moduleof claim 16, further comprising a high power, implantable medical deviceelectrically coupled to the power bus.
 18. A transcutaneous energytransfer system, comprising: a transcutaneous energy transfer primarycoil adapted to be placed outside a patient for providing a time-varyingmagnetic field that passes into the patient; a transcutaneous energytransfer device secondary coil module adapted to be implanted within thetime-varying magnetic field within the patient provided by the primarycoil for receiving and conditioning power from the time-varying magneticfield, the secondary coil module comprising: a transcutaneous energytransfer secondary coil comprising an inner winding and an outerwinding, the secondary coil configured to receive the time-varyingmagnetic field provided by the transcutaneous energy transfer primarycoil and convert the time-varying magnetic field into a high voltage,alternating current electric signal within the secondary coil; and powerconditioning circuitry disposed in a central region of the inner andouter windings and electrically coupled to the secondary coil, the powerconditioning circuitry including electronics for converting a highvoltage, alternating current electric signal from the secondary coilinto a high power, low voltage direct current electric signal, wherein amagnetic field generated by the inner winding and a magnetic fieldgenerated by the outer winding when an electrical current is applied toat least one of the inner and outer windings are substantially opposedto one another at the central region; and an implantable medical deviceelectrically coupled to the power conditioning circuitry and receivingtherefrom the high power, low voltage direct current electric signal.19. The system of claim 18, wherein the magnetic fields generated by theinner and outer windings substantially cancel one another at the centralregion.
 20. The system of claim 18, wherein the inner and outer windingsare configured to maintain a total inductive heat dissipation from thepower conditioning circuitry below about 150 milliwatts.
 21. The moduleof claim 18, wherein the medical device is a distributed medical deviceincluding a plurality of implanted modules requiring power.
 22. Themodule of claim 18, wherein the implantable medical device is a bloodpump.
 23. A method for reducing inductive heating in a transcutaneousenergy transfer system, comprising a transcutaneous energy transferprimary coil adapted to be placed outside a patient for providing atime-varying magnetic field that passes into the patient, atranscutaneous energy transfer device secondary coil module adapted tobe implanted within the time-varying magnetic field within the patientprovided by the primary coil for receiving and conditioning power fromthe time-varying magnetic field, and power conditioning circuitrydisposed in a central region of the secondary coil for converting a highvoltage, alternating current electric signal from the secondary coilinto a high power, low voltage direct current electric signal, themethod comprising: constructing the transcutaneous energy transfersecondary coil as an inner winding and an outer winding, the secondarycoil configured to receive the time-varying magnetic field provided bythe transcutaneous energy transfer primary coil and convert thetime-varying magnetic field into a high voltage, alternating currentelectric signal within the secondary coil; and selecting a diameter forthe inner and outer windings and a number of turns for each winding suchthat magnetic fields in the central region substantially cancel eachother.
 24. The method of claim 23, further comprising implanting thesecondary module in a patient.